Emissions Light on Code PO449

Hi from Texas! I am a woman and know NOTHING about cars and have a problem with my 2005 Silverado.

The emissions light went on and off for a few months so I thought it was just the gas cap. Then about a month ago, it came on and did NOT go off. When I got an oil change, the guy did a basic diagnostic that said
P0 449. For an additional $89.99, he can do a in depth diagnostic but he suggested I change the gas cap first and hope that worked.

Well, it didn't. It has to be inspected this month and my husband has been unemployed for almost 2 years (we are senior citizens). Money is tight. Does anyone know what a code PO449 is?